22 STIGMARIA FIC01DES. 



in the size of the roots continued to progress is obvious, since it is clear that if 

 the superficial layers of the bark had been successively thrown off until the 

 parenchyma wholly disappeared, the meristem zone, d\ must have been the next 

 to perish, which would apparently have arrested all further growth in the thickness 

 of the bark ; a result the occurrence of which does not seem to have taken place so 

 long as the trees continued to live. That the more superficial portions of the 

 parenchyma were so cast off is suggested by the gradual disappearance of the 

 rootlet-scars so conspicuous in Plate III, figs. 5 and 6, but which are either entirely 

 wanting, as in Plate I, figs. 1, 2, and Plate II, fig. 3, or very feebly represented in 

 the extremities of Plate II, fig. 4, and Xylograph 1. 



The Rootlets and Rootlet Bundles. 



The Lower Coal-Measures of Lancashire and Yorkshire have furnished nearly 

 all the trustworthy knowledge we possess respecting these organs. That know- 

 ledge, however, is now so complete that but little remains to be added to it. 

 Some little new light was thrown upon the subject in my Memoir, Part II, Plates 

 XXIX and XXX ('Phil. Trans.'). But since that Memoir was published much 

 additional information has been obtained. 



Orientation of the Rootlet Bundles.' 



The longitudinal section of the zylem cylinder, Plate VI, fig. 9, illustrates this 

 subject. All the vessels of special narrow radial laminse of the cylinder, from their 

 innermost to their outermost margins, are suddenly deflected outwards, as at/, to be 

 prolonged through the cortical zone as a rootlet-bundle. When the exogenous zone 

 of a very young root is in process of development, as in Plate VI, fig. 9, its outer- 

 most and newest formed Tracheids follow the course pursued by the older vessels 

 on which they rest, and reappear in corresponding relative positions in transverse 

 sections of each bundle as will be shown at page 31. As this bundle proceeds 

 outwards through the vascular cylinder, it increases in size transversely as well as 

 vertically. The formation of the bundle begins in the deflection of a solitary vessel, 

 which is the innermost and youngest one of a single radiating vascular lamina, 

 hence it is consequently a vessel of small diameter. As newer additions of larger 

 vessels are made exogenously to the vascular cylinder, the portion of each new 

 growth that is in the radial line of the lamina originating the bundle contributes 



1 M. Eenault says that these bundles " prennant naissance vers l'extremite interne des coins de 

 bois," ' Cours de Botanique fossile,' premiere annee, p. 156. This is true so far as the commence- 

 ment of their formation is concerned ; but, as will be shown in the text, they continue to arise from the 

 entire radius of that vascular wedge to whatever diameter it may attain. The erroneous notions of 

 Professor Goeppert and Mr. Binney on this question have been already dealt with at p. 12. 



